1. Field of the Invention
The present invention relates to an optical network based on wavelength division multiplexing optical transmission technology and, more particularly, to an apparatus and method for protection switching of an optical channel at each node in an optical network.
This work was supported by the IT R&D program of Ministry of Information and Communication (MIC)/Institute for Information Technology Advancement (IITA) [2006-S059-02. ASON based Metro photonic cross-connector technology].
2. Description of the Related Art
Wavelength division multiplexing (WDM) optical transmission technology has been rising as a solution to the sharp increase in demand for transmission capacity. The WDM optical transmission technology makes it possible to simultaneously transfer a plurality of wavelength channels through a single optical fiber. For example, assuming that one wavelength channel transfers 50 wavelength channels at a rate of 10 Gb/s, the total transfer rate amounts to 500 Gb/s. As can be seen from this example, the WDM optical transmission technology is very useful in high-capacity data transmission.
Meanwhile, in order to increase the efficiency and flexibility of an optical network that puts the WDM optical transmission technology into practice, technology for adding and dropping a wavelength channel at a network node is required. Such a requirement is fulfilled by fixed optical add-drop multiplexer (FOADM) technology. Furthermore, reconfigurable optical add-drop multiplexer (ROADM) technology makes it possible not only to increase the efficiency of the optical network but also to make a more economical use of network resources, etc. The use of the ROADM technology allows a certain channel to be added or dropped at a certain node, so that the network can be operated with higher efficiency.
Meanwhile, in order to further increase the efficiency of the ROADM technology, along with the development of the ROADM technology, it is predicted that an optical network topology will gradually develop from a simple point-to-point or ring type to a complicated mesh type.
However, due to its high transfer rate of 2.5 Gb/s to 40 Gb/s and the transmission of high-capacity data, situations may become serious when the signal quality of optical channel is degraded during the transmission. In order to solve this problem, an increasing interest is taken in the problems associated with protection switching of the optical channel, and furthermore it is essential to find a solution to this problem. The optical network topology of a simple point-to-point or ring type, which is commonly used at present, employs a 1+1 protection switching method. If a signal is not transmitted through one of optical fibers due to cutoff of the optical fiber or problems with, for example, an optical amplifier, another signal transmitted through the other optical fiber is detected and processed. In other words, a main signal and a sub signal are set on the basis of a signal transmission system. Thus, when the main signal has a problem with the transmission, the sub-signal is processed.
Hereinafter, reference will be made about conventional protection switching of an optical channel signal in the optical network that puts into practice the WDM optical transmission technology having this technical background with reference to FIGS. 1 through 3.
FIG. 1 illustrates the configuration of a ring-type optical network in which a conventional method for protection switching of an optical channel is realized.
As illustrated in FIG. 1, four nodes A, B, C and D interact to transmit and receive data. Each node is connected to neighboring nodes through two optical fibers P and W. One of the optical fibers P and W is a working fiber W as a main optical fiber, through which traffic runs in a clockwise direction. The other optical fiber P is a protection fiber, through which traffic runs in a counterclockwise direction when the working fiber W has any problem.
For example, in the case of traffic proceeding from the node B to the node A, a transmitter of the node B splits a signal into two identical signals, and then transfers one of the two signals through the working fiber W and the other signal through the protection fiber P. A receiver of the node A has a switch to select data received through the working fiber W and the protection fiber P. As illustrated in FIG. 1, the receiver of the node A receives the signal through the working fiber W in a normal condition. However, when the working fiber W is partly cut off, the receiver of the node A may receive the signal through the protection fiber P. This technique is known as a unidirectional path switched ring (UPSR). More specifically, the traffic, which is transmitted through one transmission path, is additionally transmitted using another transmission path, so that the signal can be transmitted to its destination even when the signal fails to be transmitted through one transmission path.
FIG. 2 is a block diagram illustrating the configuration of a conventional apparatus for protection switching of an optical channel. As illustrated in FIG. 2, the conventional optical channel protection switching apparatus includes a splitter 200 for splitting a signal, which is input in order to conduct the aforementioned 1+1 protection switching, into multiple signals, a switch 210, and optical transponders 220a and 220b. Each of the optical transponders 220a and 220b includes an optical transmitter, an optical receiver, and other additional elements, and performs optical/electrical and electrical/optical signal conversion, that is, converts an optical signal to an electrical signal, and an electrical signal to an optical signal.
First, when transmitting the signal, the splitter 200 splits an input electrical signal into two electrical signals and outputs the two electrical signals to the optical transponders 220a and 220b. The optical transponders 220a and 220b convert the input electrical signals to optical signals. The optical signals are transmitted to another node through the working fiber P and the protection fiber P, respectively, via a device, such as ROADM or FOADM, or a photonics cross-connect switch (PXC).
Further, the optical signals, which are received from the other node through the working fiber P and the protection fiber P, are converted to electrical signals by the optical transponders 220a and 220b, and then the converted electrical signals are output. At this time, the switch 210 selects the signal received through the working fiber W in a normal operation. However, when the working fiber W is not in a normal condition, the switch 210 selects the signal received through the protection fiber P.
FIG. 3 is a block diagram illustrating the configuration of another conventional apparatus for protection switching of an optical channel. As illustrated in FIG. 3, the conventional optical channel protection switching apparatus includes an optical transponder 300, a splitter 200 and a switch 210. The optical transponder 300 converts an electrical signal to an optical signal, and outputs the optical signal. The splitter 200 splits the optical signal, which is output from the optical transponder 300, into two identical signals, and then transmits the two signals through the working and protection fibers W and P, respectively. The splitter 200 may be either an optical divider or an optical coupler. The split signals are transmitted to another node through the working fiber W and the protection fiber P, respectively, via a device, such as ROADM or FOADM, or a photonics cross-connect switch (PXC).
Further, the switch 210 selects one of the optical signals, which are received from the other node through the working and protection fibers. During normal operation, the switch 210 selects the signal received through the working fiber W. However, when the working fiber W is not in a normal condition, the switch 210 selects the signal received through the protection fiber P. The optical transponder 300 converts the optical signal, which is selected by the switch 210, to an electrical signal, and then outputs the electrical signal.
The conventional protection switching of the optical channel signal has been described above with reference to FIGS. 1 through 3. The protection switching of the optical channel signal as described above may be applied to an existing point-to-point or ring topology. However, as already mentioned above, the optical network topology is predicted to become more complicated, and ultimately develop into the mesh topology.
FIG. 4 illustrates the configuration of a mesh-type optical network. Twelve nodes A through L are connected to one another, each of which is connected to neighboring nodes through two optical fibers. In the case of a point-to-point or ring type optical network, each node is connected with two neighboring nodes and thus has two inputs and two outputs for the optical fibers. However, in the case of the mesh-type optical network, each of the nodes A, D, I and L has two inputs and two outputs, each of the nodes F and G has four inputs and four outputs, and each of the nodes B, C, E, H, J, and K has three inputs and three outputs. In other words, in the case of the mesh-type optical network, the number of the inputs and the number of outputs vary depending on the position of the node. Thus, the optical channel protection switching apparatus, in which each node has two inputs and outputs of the optical fibers as illustrated in FIGS. 2 and 3, can no longer be applied to the mesh-type optical network. Furthermore, the mesh-type optical network can set various traffic paths, where intermediate nodes may be included in more traffic paths. However, one or more protection fibers may be provided in addition to the working fiber, causing a wastage of resources.